1. Field of the Invention
The present invention relates to a control method and control apparatus for a feed system which comprises a drive mechanism for moving a slide along a straight line, a servo motor forgiving power to the driving mechanism, a rotational position detection means for detecting the rotational position of the servo motor, and a position detection means for detecting the position of the slide.
2. Description of the Prior Art
A feed system provided in an NC machine tool or the like comprises a drive mechanism for moving a slide such as a machine table along a straight line, a servo motor for giving power to the drive mechanism, a rotational position detection means for detecting the rotational position of the servo motor, etc. The drive mechanism comprises, for example, a ball screw (lead screw) driven by the servo motor, a nut screwed onto the ball screw, etc. and causes the table to move along the ball screw by means of the nut moving in the axial direction of the ball screw with the rotation of the ball screw.
The servo motor is feedback controlled by a control unit; in the case of a semi-closed system, the rotational position of the servo motor (equivalent to the position of the slide) and the rotational speed of the servo motor (equivalent to the feed speed of the slide) are feedback controlled based on the data detected by the rotational position detection means. On the other hand, in the case of a system employing full-closed feedback control, a position detection means such as a linear scale is included in order to detect the position of the slide, and the position control is performed based on the position data detected and fed back by the position detection means, while the speed control is performed based on the speed data detected and fed back by the rotational position detection means.
Here, a gap, or backlash, inherently exists between the thread grooves of the ball screw and nut and the balls constrained between them. There also occurs a lost motion due to the deflection of the ball screw. Accordingly, in the case of the semi-closed system that performs the position control by reference to the rotational position of the ball screw, when the direction of rotation of the ball screw, and hence the direction of movement (feed direction) of the slide, is reversed, a positioning error occurs by an amount equal to the amount of backlash and the degree of deflection of the ball screw. In the prior art semi-closed system, therefore, it has been practiced to detect the backlash amount in advance and to correct the position by an amount equal to the detected backlash amount when reversing the feed direction of the slide. In the full-closed feedback control, on the other hand, the positioning error due to the backlash or the deflection of the ball screw does not occur, since the control is performed by directly detecting the position of the slide.
Further, in both the semi-closed system and the full-closed system, when reversing the feed direction of the slide, a delay equal to the amount of backlash occurs in the reversing action of the slide (a delay in tracking the target position). In a machining center, when the tool is moved from one quadrant to another, for example, during a circular arc cutting, the above delay has resulted in the formation of a bump when cutting an outer circumferential arc, and a recess when cutting an inner circumferential arc. In the prior art, therefore, in order to obtain a smooth arc surface free from bumps or recesses, when reversing the feed direction of the slide it has been practiced to increase the feed speed during direction reversal (including the starting speed) in accordance with the backlash amount, thereby reducing the delay in tracking the target position.
The amount by which the feed speed is increased has been determined empirically by actually performing cutting operations, or based on correlation with the backlash amount. In the semi-closed system, the backlash amount is calculated from actual measurements, while in the full-closed system, the backlash amount is calculated from a difference between the position data of the slide detected by the position detection means and the position data of the slide calculated based on the rotational position data detected by the rotational position detection means.
However, the backlash amount is not constant, but increases with age due to the wear of the thread grooves of the ball screw and nut or the wear of the balls. As a result, if the amount by which the feed speed is to be increased during the reversal is left at its original setting, the backlash amount increasing with age cannot be accommodated and highly precise position control of the slide cannot be accomplished, thus leading to the problem that the bump or recess that occurs when switching from one quadrant to another gradually increases in size. Furthermore, if the pretension of the ball screw changes due to thermal expansion, the rigidity of the ball screw changes, and this change in rigidity causes the lost motion to vary. This also results in the problem that the bump or recess that occurs when switching from one quadrant to another changes in shape. In a machine tool designed to operate in a semi-closed control mode, this has lead to the problem that the accurate backlash amount and lost motion amount have to be re-measured periodically, that is, such time and labor consuming work has to be done periodically if highly precise machining is to be achieved.
On the other hand, in a machine tool designed to operate in a full-closed control mode, the above problem does not occur since the backlash amount and lost motion amount can be calculated from the position data detected by the position detection means and the rotational position data detected by the rotational position detection means, but in this case also, there arises the following problem.
That is, since the ball screw tends to thermally expand due to the heating of the screw supporting bearings and the frictional heat generated by the balls running in the thread grooves, and the balls tend to suffer deformation due to load, the backlash amount calculated from the position data detected by the position detection means and the rotational position data detected by the rotational position detection means, as described above, is inaccurate as it contains effects due to the thermal expansion of the ball screw and the deformation of the balls in addition to the actual backlash amount. As a result, if the amount by which the speed is to be increased during feed direction reversal is set based on such inaccurate backlash amount, highly precise position control of the slide cannot be accomplished.
In view of the above situation, it is an object of the present invention to provide a feed system control method and control apparatus that can calculate an accurate backlash amount in a periodic or non-periodic manner and can accomplish highly precise position control.
The present invention which solves the above problem concerns a feed system control apparatus and control method for controlling the operation of a feed system which comprises a drive mechanism for moving a slide along a straight line, a servo motor for giving power to the drive mechanism, a rotational position detection means for detecting the rotational position of the servo motor, and a position detection means for detecting the position of the slide, wherein the control apparatus and control method are provisioned to perform the processing of:
calculating position errors before and after a reversal in direction of movement of the slide, each from a difference between the position data of the slide detected by the position detection means and the position data of the slide calculated from the rotational position data of the servo motor detected by the rotational position detection means;
subsequently, calculating a backlash amount for the drive mechanism from a difference between the calculated position error before the reversal and the calculated position error after the reversal; and
when thereafter reversing the direction of movement of the slide, increasing feed speed during direction reversal in accordance with the calculated backlash amount.
According to the present invention, positions errors are calculated before and after the reversal of the feed direction of the slide, each from the difference between the position data of the slide detected by the position detection means and the position data of the slide calculated based on the rotational position data detected by the rotational position detection means, and then the backlash amount of the drive mechanism is calculated from the difference between the position error before the reversal and the position error after the reversal.
Since the ball screw tends to thermally expand due to the heating of the screw supporting bearings and the frictional heat generated by the balls running in the thread grooves, and the balls tend to suffer deformation due to load, as earlier described, the position error calculated from the difference between the position data detected by the position detection means and the rotational position data detected by the rotational position detection means contains error factors such as the thermal expansion of the ball screw. In the present invention, however, since the position errors are calculated before and after the reversal of the slide respectively, and the backlash amount is calculated from their difference, the backlash amount can be calculated accurately, canceling the error factors such as the thermal expansion described above.
Thereafter, when reversing the direction of movement of the slide, the feed speed (including the starting speed) is increased in accordance with the thus obtained accurate backlash amount. Thus, according to the present invention, it becomes possible to control the position of the slide highly precisely.
Preferably, the position error data before the reversal is taken immediately before the reversal, and the position error data after the reversal is taken immediately after the reversing action is completed. Further, the backlash amount is calculated continually at periodic or non-periodic intervals. As a result, if the backlash amount increases with age due to the wear of the thread grooves of the ball screw and nut or the wear of the balls, the position of the slide can be controlled highly precisely, without requiring time and labor consuming work.
The control apparatus, more specifically, comprises: a parameter storage section which stores a parameter that has been set in accordance with the backlash amount of the drive mechanism; a backlash acceleration executing section which reads out the parameter stored in the parameter storage section, and increases the feed speed during the direction reversal in accordance with the parameter when reversing the direction of movement of the slide; a backlash amount calculation section which calculates the position errors before and after the reversal of the direction of movement of the slide, each from the difference between the position data of the slide detected by the position detection means and the position data of the slide calculated from the rotational position data of the servo motor detected by the rotational position detection means, and subsequently, calculates the backlash amount for the drive mechanism from the difference between the calculated position error before the reversal and the calculated position error after the reversal; a backlash acceleration amount calculation section which, based on the backlash amount calculated by the backlash amount calculation section, calculates the acceleration amount to be applied when reversing the direction of movement of the slide; and a parameter setting section which, based on the acceleration amount calculated by the backlash acceleration amount calculation section, sets a parameter appropriate to the acceleration amount, and updates the parameter stored in the parameter storage section with the set parameter.